How to Mirror Keyframes?
Mirroring a Keyframe is pretty easy. Go to "pose mode" and you must select all your bones including the ones you want to copy it to, then over in the right side of your screen will be three buttons



The first one is Copy

second Paste on same side

And the third is Paste on opposite side

With all your bones still selected press the first button to copy your pose you wish to be on the other side, then place it in the desired spot on the timeline by clicking the third button . This will mirror it to the left side(or opposite side) of your mesh.

If you want it a keyframe just click insert keyframe( with all bones still selected)